In forming sheets of material, such as plastics, paper or other materials, the materials generally are formed in a continuous process as a moving film or web. In order to most effectively monitor and control the process, one or more properties of the web must be determined while the web is in motion. The properties which are of interest here are termed "thickness related properties" and include in particular circumstances web thickness, caliper, density, basis weight, or others. Further, in many cases it is important to measure web profile variations, such as streaks and hence the web property needs to be measured across the width of the moving web.
To measure the desired web property, one or more sheet sensors are employed in fixed or web traversing structures. The moving web can be constrained to move over a fixed reference surface, but more commonly the moving web is unsupported in the measurement region and is subject to deviations from the nominal path through the measurement region. The nominal path through the measurement region is usually referred to as a "pass-line", and deviations from that pass-line such as "flutter" or other lower frequency deviations alter the pass-line of the web through the measurement region.
A desirable method of forming a measurement region across the width of the web, is to utilize nuclear radiation in a traveling sensor arrangement which traverses the moving web to measure the desired web property. A radioactive thickness/density gauge is based upon the principle that a mass of material will absorb the products of radioactive emission in a known and repeatable manner. An industrial web gauging instrument, known as a beta-gauge, typically utilizes a radioactive isotope which decays through beta particle emission. The radioactive isotope is mounted in an enclosed head or source, which projects the radiation through the web to a second head or detector which includes a radiation detector. The amount of radiation sensed by the detector is directly related to the amount of radiation absorbed by the web material being measured.
The absorption of beta particles by matter or material is shown by the following equation: EQU N=N.sub.o (1-e.sup.-.alpha.d)
where N is equal to the amount of beta radiation absorbed by the material, N.sub.o is the amount of beta radiation incident on the material prior to absorption, .alpha. is a constant related the type of material (which primarily is a function of the material density) and d is the thickness of the material. The signal generated by the detector thus can be utilized in an instrument to accurately and repeatedly solve for the material thickness.
The relative position of the moving web in the measurement region space between the source and detector heads is called the web pass-line. Due to the nature of beta particle interaction with the web material, specifically due to the angular dispersion of the beta particles as they pass through the web, the amount of radiation sensed by the detector varies with pass-line deviations. Since the fluctuating web pass-line can vary significantly and at a high frequency during a measurement of the moving web, the accuracy of the gauge or sensor in part depends on the ability to compensate for pass-line variations.
The prior art has compensated for pass-line variations in a variety of manners. One prior art compensating method is to employ a radiation absorption shading strip or disc on the detector to geometrically flatten the radiation intensity profile entering the detector. The shading strip in effect smooths the radiation profile, thus making the source-detector geometry less sensitive to pass-line variations. However, it is clear that this approach, while it reduces the sensitivity to pass-line variation, also reduces the sensitivity of the system. The result is reduction in the ability of the measuring system to resolve small changes in moving web thickness, or the ability to identify the presence of small streaks on the moving web. Further, the shading strip or disc also requires precise alignment between the source and detector in both the initial setup of the gauge and during transverse scanning of the moving web. The alignment tolerance can be on the order of .+-.0.005 inches, which tolerance is virtually impossible to maintain across the moving web width, which typically is a width of sixty (60) inches. This results in significant inaccuracy in the thickness measurements of the moving web.
Another approach to minimizing the effect of pass-line variations is to simply reduce the speed of response of the sensor, thereby obtaining a slowly responding average of the target thickness. While that approach also reduces the effect of pass-line variation, it is clear that it does so at the expense of streak resolution and the ability to respond to small thickness variations in the web.
Another prior art, for example, is Watson U.S. Pat. No. 4,276,480, which discloses various embodiments of optical sensors for measuring web thickness, and utilizes one such device in combination with a nuclear detector to produce a correction function relating nuclear sensor response to variations in pass-line position. The approach suggested in the '480 patent is deficient for a number of reasons, a primary reason being the rather complicated optical system utilized in illuminating the web with a spot of intense radiation, then sensing the change in spot position resulting from deviation in web position. The optical system includes a rather complex optical path in order to allow for the generation of light, its deflection onto the web, its deflection from the web and its collection on a linear detector all in a single housing. The optical system also suffers from additional defects in an industrial environment of being susceptible to erratic operation if not maintained adequately clean, as well as erratic operation that can result when producing a web of non-uniform reflectivity. In short, it does not appear that the proposed device is adequate in all respects to meet the demands of the industrial environment.
It would be desirable to reliably eliminate pass-line variations from the moving web measurements in the industrial environment, while still providing sensitivity to small variations in the moving web thickness and sensitivity to streaks on the moving web.